1. Field of the Invention
The present invention relates to a method and apparatus for mounting and thinning a wafer. In particular, the present invention relates to a method and apparatus for mounting a bumped wafer to a wafer mounting chuck and thinning the wafer to a predetermined thickness.
2. State of the Art
Typically, in a manufacturing process, a plurality of integrated circuits is simultaneously patterned and defined on the front surface of a single silicon wafer. The circuits are generally aligned in rows and columns in an orthogonal format. After the integrated circuits are fully defined, the wafer is diced by a singulation machine along lines between the rows and columns, separating the wafer into a plurality of individual integrated circuit dice. The integrated circuit dice can then be secured within individual packages and/or incorporated into electronic devices.
In the typical manufacturing process, the silicon wafer is sliced from a generally cylindrical ingot. The wafer is at first sliced sufficiently thick so as not to warp or break during the various manufacturing processes. However, in some instances, the desired thickness for the finished dice is less than the initial thickness of the sliced wafer. Therefore, after the integrated circuit patterns are formed on the wafer, it has been necessary to grind the back surface of the wafer to reduce its thickness as desired for the individual integrated circuit die.
Grinding machines for grinding down the back surfaces of silicon wafers are known in the art. The known machines have chuck tables for securing a plurality of wafers in position to be ground by one or more grinding wheels. Examples of such grinding machines are illustrated in U.S. Pat. No. 5,679,060 (Leonard), U.S. Pat. No. 4,753,049 (Mori), U.S. Pat. No. 5,632,667 (Earl), and U.S. Pat. No. 5,035,087 (Nishiguchi).
Currently available wafer processing systems are unsatisfactory, particularly for grinding wafers after the contact pads of the integrated circuits thereon are bumped, known as bumped wafers. Recently, the market demands the thinning of wafers to about 6 mils or less for chips utilized in ultra-compact applications such as in cell phones. For example, see U.S. Pat. No. 5,476,566 (Cavasin), which discloses a method for thinning wafers by adhesively attaching the wafers to a supporting substrate, but does not disclose thinning wafers after being bumped. Also, U.S. Pat. No. 6,162,703 (Muntifering et al.), assigned to the assignee of the present invention, discloses a method for thinning and singulating dice from an unbumped wafer by adhesively attaching the unbumped wafer to a table and precutting notches in the unbumped wafer prior to the thinning thereof. However, for bumped wafers, it is necessary to thin the wafer after bumping because, currently, the wafer must be at least 12 mils thick to undergo the bumping process without the likelihood of damage thereto. Further, it is important that the wafer be held tightly in place during the thinning process, typically with a vacuum chuck.
Vacuum chucks include a series of apertures in the surface of the chuck to which a vacuum source is connected. The suction created between the surface of the chuck and the bottom of the wafer securely holds the wafer in place. For example, see U.S. Pat. No. 6,120,360, assigned to the assignee of the present invention, which discloses a vacuum chuck made for securing to the planar face surface of a wafer. However, the vacuum chuck is segmented into quarters and also requires the wafer to be quartered, resulting in additional process steps and potential for error in handling four times the number of wafer parts per wafer.
Although vacuum chucks perform very well for wafers having a planar face surface through which air cannot pass, such vacuum chucks will not work well for a bumped wafer. Specifically, the required suction force between the surface of the chuck and the active surface of the wafer cannot be achieved since the suctioned air will pass through the gap provided by the bumps formed on the bond pads of the integrated circuits formed on the surface of the wafer. To overcome such problems, vacuum chucks for bumped wafers are typically made to provide the suction on the active surface's periphery where there are no bumps. However, such vacuum chucks do not provide the necessary suction at the wafer's periphery for effectively holding a bumped wafer for the thinning thereof because there is not enough surface area proximate the wafer's periphery without the integrated circuits and bumps thereon. As a result, it has been suggested to increase the area proximate the wafer's periphery without the integrated circuits and bumps formed on the bond pads thereof to provide greater suction on the wafer. However, this would unacceptably limit the number of bumped dice per wafer, thereby resulting in a reduction of yield.
Therefore, it would be advantageous to provide a method and apparatus for thinning bumped wafers that provide the necessary area for suction without limiting the number of bumped dice on the wafer.